1. Field of the Invention
This invention relates to structural materials associated with thermal electric power plants requiring the use of a heat-resisting steel. More particularly, it relates to steam turbine rotors for use in thermal electric power generation, and forged steel products for use in electric power generation.
2. Description of the Related Art
Among heat-resisting steels used in steam turbine plants for electric power generation, high-temperature turbine rotor materials include CrMoV steel and 12Cr steel. Of these, the use of CrMoV steel is restricted to plants having a steam temperature up to 566.degree. C. because of its limited high-temperature strength. On the other hand, rotor materials based on 12Cr steel (e.g., those disclosed in Japanese Patent Publication (JP-A) No. 40-4137/'65 and the like) have more excellent high-temperature strength than CrMoV steel and can hence be used in plants having a steam temperature up to 593.degree. C. However, if the steam temperature exceeds 593.degree. C., such rotor materials have insufficient high-temperature strength and can hardly be used for steam turbine rotors.
Now, 12Cr steel is explained in greater detail. The term "12Cr steel" refers to a group of materials which originated from a heat-resisting steel developed in England and actually having a Cr content of 12%. In the composition of this group of materials, the contents of alloying elements have been increased every year in order to improve high-temperature strength, and the segregation of alloying elements has become manifest as the size of stocks is increased. For these and others reasons, the present situation is such that the formation of .delta.-ferrite may occur unless the content of Cr is reduced. Although the content of Cr may recently be as low as about 8%, this group of materials is nominally designated by "12Cr steel" in its broad sense, because the content of Cr was 12% at the initial stage of development. However, the actual content of Cr in these materials ranges from 8 to 13%. Among these materials, those having a Cr content of 9% or less may be also referred to as "9%Cr steel".